Televisions contain a number of components including a screen such as a cathode ray tube (CRT), liquid crystal display (LCD) or plasma display, as well as circuitry to receive a signal to be displayed and display the signal on the screen. This circuitry includes a tuner that tunes to a particular carrier (frequency) to receive the desired signal.
Television tuners receive carriers over a predetermined range, e.g. from 50 MHz to 860 MHz over some standards. This is a relatively wide range of frequencies compared to other electronic devices, such as cellular telephones, which operate over a frequency range of 100 MHz or so. The incoming signal power to a tuner can vary by many orders of magnitude depending on a variety of factors such as distance of the tuner to the signal source or the environmental conditions, for example. Moreover, the signal strength may vary continuously if the television is mobile as the television is being transported.
However, the change in signal strength is problematic as before the signal is provided to an output of the tuner, it is amplified by an amplifier having a constant gain. In this case, the signal strength provided to the amplifier is adjusted so that the signal is within the range of the amplifier. That is, if the amplitude of the signal is too large, the amplifier will distort the peaks of the signal and thus degrade the output of the tuner. Accordingly, there exists a need to continuously control the gain of the tuner such that a constant output power level is achieved.
To control the gain of the tuner, a variable attenuator is usually provided between the input signal and the amplifier. It is desirable to have a continuous gain control using an analog control voltage. Likewise, it is desirable to have the slope of the gain linear in dB/V. However, the gain variability function adds complexity, noise, and distortion problems. For example, a tradeoff exists between the sensitivity of the attenuation control voltage and the attenuation range. For increasing attenuation ranges, it becomes more difficult to maintain a linear in dB/V gain slope.